EN 820-4-2009 pdf download.Advanced technical ceramics – Thermomechanical properties of
monolithic ceramics – Part 4: Determination of flexural creep deformation at elevated temperatures.
leading to acceleration of creep rate and the linking of damage to form cradcs (creep rupture). In some circumstances it is not possible to distinguish the mechanism of failure. In either case, the test piece lifetime under the imposed temperature and stress conditions can be an important aspect of a matenals performance.
The analysis given in this European Standard (see 8.6) produces purely nominal data, assuming that the actual maximum nominal stress in the test piece is linearly proportional to the test force applied and Is constant dunng the test. Moreover, an additional assumption of linear dependence of strain on stress is made for some deflection measurement methods. Furthermore, it does not give engineering creep data equivalent to separate pure tensile or compressive conditions. In many cases, the creep rate dependence is to the maximum stress, and can differ in tension and compression, Typically, the true maximum stress in the test piece is less than that calculated using Equation I because of faster relaxation at higher stress levels, and the true surface strain rate can be greater than a linear prediction in certain geometrical arrangements for determining the deformation, particularly if this is done using the relative displacement of the loading system. The Bibliography contains references to more detailed theoretical analyses of flexural creep accounting for such non-lineanties.
5 Principle
The method Involves supporting a bar test piece on two supports near its ends, heating it to the required elevated temperature which is maintained constant, applying a force to two loading points spaced symmetrically between the support points, and recording the deflection of the test bar with time.
The deflection of the test piece is measured indirectly and continuously or at appropriate time intervals during the test using the displacement of the loading system (see e.g. Figure la), or by using contacting extensometer rods at given positions on the test piece (see e.g. Figures lb to le). The Indirect measurement of deflection (Figure la) is converted into a nominal maximum surface strain in the test piece assuming a linear relationship between stress and accumulated strain. Similar assumptions are involved in analysing deflections between support points and the span centre (Figure lb and ic). When employing displacement measurement between the loading points and the span centre (Figures Id and le). the analysis assumes uniform curvature of the test piece, a linear relationship between strain and distance from the neutral axis, and equal behaviour in tension and compression. The slope of the strairiltime curve can be converted to a creep strain rate.
6 Apparatus
6.1 Creep test loading jig
The test jig is essentially a four-point bend flexural test jig similar to that described In EN 820-1 for flexural strength testing at elevated temperatures. It comprises a pair of parallel 5 mm diameter support rods positioned 40 mm apart on a refractory supporting structure. These rods shall be free to roll to eliminate friction effects. In contrast to the articulating requirement in EN 820-1, articulation is not required provided that the rods are accurately parallel in the horizontal plane to within 0,001 mm per mm length of rod.
The loading assembly comprises a similar pair of freely rolling rods positioned on a loading block. The spacing between these rods shall be between 30 % and 50 % of the spacing of the support rods. The loading block shall be free to articulate relative to the loading column in order to permit alignment of the loading rods on the test piece upper surface.
NOTE 1 Subject to agreement between parties, other test piece support and loading spans can be employed. This can be particulaily advantageous for creep-resistant materials. In addition, in some conditions It is recognized that freely rolling rollers, although preferred. may not be feasible. Such deviations from this method should be reported. The effect of restricted roller rotation may or may not be significant depending on the test material and the testing conditions. There Is some evidence to suggest that the surface of glass-phase containing materials, or materials which oxidize to give a viscous glassy surface layer, can have a low coefficient of friction against the roller material at the test temperature, such that over the period of the test any friction becomes negligible. However, this situation cannot always be guaranteed
The loading block shall be guided appropriately such that the loading rods are positioned mid-way between the support rods, thus centrally loading a test piece when placed on the support rods.
The parts of the loading jig shall be constructed from a ceramic material which is anticipated to be more reStstant to deformation than the materials under test. In addition the support and loading rods shall be of a material which does not chemically react with the test piece.
NOTE 2 Suitable materials indude high-purity alumina for use with most oxide-based test pieces, or sintered silicon carbide for most non-oxide ceramics.
NOTE 3 Test jig parts manufactured from sinlered sihcon carbide or other silicon-based non-oxide ceramics develop oxidation films in a short period of time when exposed to temperatures typically above 1 300 C in an oxygen-containing atmosphere. This can cause prevention of rolling of rollers and impairment of jig function
6.2 Heating device
A heating device surrounds the loading jig in such a manner as permit access to the jig for the purposes of mounting and demounting test pieces. The heating device shall be capable of maintaining a constant test piece temperature to ± 3 C over the duration of the test.
The temperature of the test piece shall be recorded using a thermocouple manufactured in accordance with EN 60584-2 allowing the use of reference tables in EN 60584-1 or, alternatively calibrated in a manner traceable to the International temperature scale ITS-90. The tip of the thermocouple shall be close to but not touching the test piece. It shall previously have been determined that the temperature of the test piece does not vary by more than ± 3 C over its length when temperature has stabilized for more than 30 mm.
The heating device can incorporate or be incorporated within a vacuum or other appropriate chamber for control of gas atmosphere if appropriate to the determination.EN 820-4-2009 pdf download.